As Tony said, it doubles the following consonant. So words like "gakkou" (school) would look like がっこう. The っ 'sounds' like a stop, somewhat. Try saying the words "hot tea" and you might understand what I mean. When we say "hot tea" naturally, it sounds more like "hottea" doesn't it? That's what the っ does. ^_^

As you (just speaking to beginning Japanese students in general) get better and better at Japanese, (ESPECIALLY LISTENING! PRACTICE LISTENING.) you'll be able to tell the difference between words like "kite" (cut) "kiite" (listen) and "kitte" (come/approach). And you'll be able to hear the long 'o' sound better, and you'll be able to say "kon'yaku" and it won't sound like "kon'nyaku"....

But getting the small tsu down should be a first step. That is... after getting the pronunciation difference between "kowai" and "kawaii." :P